cc-wrapper: don't add broken cflags to clang

On non-GNU (gcc) compilers, there is no "/lib/gcc/..."
so when this is eventually expanded this is empty
resulting in an incomplete "-idirafter " that
eats the next argument:

-idirafter -B/nix/store/wamjwwdvkmhbf4f2902nhw8jxxzv0hy3-clang-wrapper-4.0.1/bin/